1. Field of the invention
This invention relates to a pressure control circuit for use in an automatic transmission of a vehicle, and more particularly to a pressure control circuit of the type described, which is adapted for use in a pressure regulating circuit for the output hydraulic pressure which operates a frictional engaging means such as the speed change gear of an automatic transmission, a clutch, a brake and the like.
2. Description of the prior art
In general, when the change-over from a forward driving mode to a backward driving mode or vice versa in a speed change gear is accomplished by means of hydraulic control, abrupt engagement of a frictional engaging means will lead to the occurrence of a shock, whereby not only a driver or passengers undergo uncomfortable feeling but also parts of a speed change gear or other associated parts will receive impact, resulting in the failure to provide an intended service life therefor. Hitherto, many attempts have been suggested to remedy a shock which will be experienced upon engagement of the aforesaid engaging means. The common practice adopted for such prior art mechanism is that (i) a discharge pressure from an oil pump is adjusted by means of a pressure regulating valve to a given level which accommodates the maximum load running operation of a motor vehicle, and (ii) there are provided an orifice in the oil line from the pressure regulating valve to an actuator of a clutch or a brake for suppressing or smoothing a pressure rise at a clutch or brake, upon actuation of the actuator, and a hydraulic pressure augmenting or increasing means such as an accumulator, a modulator valve or the like.
However, such attempts only meet partial success in solving this sort of problem, because the hydraulic pressure, which has been regulated by means of the aforesaid pressure regulating valve, i.e., the line pressure still remains high, as compared with the pressure which would be required for a clutch or brake upon their engagement in the normal running of a vehicle, so that there still remains a shock at the time of engagement. Thus, the relief of this sort of shock from a driver or passengers dictates the use of a hydraulic pressure augmenting means having a complicated construction, and hence an increase in size.